Field of the Invention
The present invention relates to upgrading refinery feedstocks, in particular to reduce the total acid number by neutralizing naphthenic acids.
Description of Related Art
Compositions of natural petroleum or crude oils vary significantly based on numerous factors, mainly the geographic source, and even within a particular region, the composition can vary. Common to virtually all sources of crude oil is the existence of heteroatoms such as sulfur, nitrogen, nickel, vanadium and others. Some crude oils also contain naphthenic acid compounds. These impurities are present in quantities that impact the refinery processing of the crude oil and its fractions.
The term “naphthenic acid” is derived from the first observation of the acidity in naphthenic-based crude. The chemical compositions of naphthenic acids are extremely complex, and a great variety of structures and compositions fall within the classification of naphthenic acids. Naphthenic acids are predominantly made up of carboxylic cycloaliphatic acids substituted with alkyl, with lower proportions of non-cycloaliphatic acids. Aromatic, olefinic, hydroxylic and dibasic acids may be present as minor components. The molecular weight of naphthenic acids present in crude oils, as determined by mass spectrometry, varies generally between about 120 and more than 700 grams per mole. Presence of naphthenic acid compounds contributes to the acidity of crude oils and is one of the major causes of corrosion in oil pipelines and distillation units in oil refineries. Consequently, crude oils with high naphthenic acid concentrations are considered to be of poor quality and are marketed at a lower price.
Total acid number (“TAN”) is a commonly accepted criterion for the oil acidity, although its correlation with corrosive behavior is still uncertain. Specifically, TAN represents the number of milligrams of potassium hydroxide required to neutralize the acidity of 1 gram of oil. Some crude oils possess extremely high levels of naphthenic acidity (e.g., oils which would require between 3 and 10 milligrams of potassium hydroxide per gram of oil in order to neutralize such acidity), and do not meet current material specifications of refineries. Metallurgic adequacy of industrial units is obtained by substituting equipment, metal pipes, etc., and is a function of naphthenic acid distribution in the oil fractions, which are subject to change in processing oils coming from new reservoirs.
The high acid content also influences the value and marketability of crude oil. For example, currently the market value is discounted for crude oil having a TAN greater than 0.5 milligrams potassium hydroxide per gram of oil.
Moreover, the polarized character of carboxyls promotes the formation of emulsions, especially in heavier hydrocarbon feedstocks. This reduces the efficiency of the desalination stage of petroleum, making the separation of the water/oil emulsions difficult. Therefore, high acidity not only reduces market value, but adversely affects the refining process.
Reducing TAN from refinery feedstocks is regarded as one of the most important processes in heavy oil upgrading. One approach is to mix feedstock having a relatively high TAN with a feedstock having a relatively low TAN. However, the acidic compounds remain in the blended feedstock, and the higher market value low TAN hydrocarbon feedstock is sacrificed. Other methods include washing a feedstock with a caustic solution. This treatment can remove naphthenic acids, but the process generates significant amount of wastewater and emulsions that are difficult to treat. Further approaches include adsorption of the naphthenic acid through adsorbent compounds with or without catalytic properties or contacting with other catalysts, including hydrotreating catalysts.
Nonetheless, a need remains in the industry for improved processes for treatment of refinery feedstocks to reduce the TAN, neutralize naphthenic acids, and/or break or prevent the formation of emulsions, in an efficient and economically feasible manner.